This invention relates generally to scanning probe systems, such as scanning probe microscopes and profilometers, and more particularly to the probe assemblies used in these scanning probe systems.
Scanning probe microscopy (SPM; also known as atomic force microscopy (AFM)) is considered a spin-off of scanning tunneling microscopy (STM). An SPM system measures the topography of a sample by scanning (sliding) a probe having a small tip over the sample""s surface and monitoring the tip position in the z-direction at each point along the scan path. Alternatively the SPM probe can be used as a nano-Spreading Resistance Probe (nano-SRP), used for the determination of the resistance and carrier profile of a semiconductor element, or for nano-potentiometry measurements of the electrical potential distribution on a semiconductor element.
FIG. 30 is a perspective view showing a conventional SPM system 40. SPM system 40 includes a movable XY stage 42 for supporting a sample 45, a probe 50 mounted to a suitable structure (holder plate) 60, a probe measurement device 70, and a computer/workstation 80 that serves as both a system controller and a measurement data processor. Holder plate 60 is movable in the z-axis direction by a suitable motor (e.g., a piezoelectric device) to selectively position probe 50 relative to sample 45. Similar motors (not shown) drive XY stage 42 in the xy-plane, thereby causing probe 50 to scan along the upper surface of sample 45, when the probe is in the lowered position. Computer 80 generates control signals that are utilized to control the movements of holder plate 60 and XY stage 42. In most conventional SPM systems, the up-and-down motion of probe 50 is detected by measurement device 70 using the so-called xe2x80x9coptical leverxe2x80x9d method, wherein a laser beam LB generated by a laser 72 shines onto a cantilever surface of probe 50, and the reflected beam hits a two- or four-segment photodiode 75. Measurement data generated by photodiode 75 is passed to computer 80, which processes the measurement data, and typically generates a magnified view of the scanned sample.
FIG. 31 shows probe 50 in additional detail. Probe 50 includes a holder chip (mounting block) 51, a straight cantilever section (stylus) 52 extending from holder chip 51, and an xe2x80x9cout-of-planexe2x80x9d tip 55 that extends perpendicular to cantilever section 52. Probe 50 is supported by holder block 60 at an angle to facilitate contact between tip 55 and an upper surface of sample 45. The choice of the materials from which holder chip 51, cantilever section 52, and tip 55 are composed depends on the type of measurement the probe is intended for. For topography measurement, a dielectric or a semi-conductive tip can be used, whereas for resistance determination and nano potentiometry require a highly conductive tip, preferably with high hardness and low wear.
One problem associated with conventional probes is that they are expensive and difficult to produce. Conventional probes are typically formed by bulk micromachining high quality, and therefore expensive, monocrystalline silicon (Si) wafers. As indicated in FIG. 31, the relatively large size of each probe 50 is due to the integrated holder chip 51, which is mounted to holder plate 60, and cantilever 52, which must extend from under holder plate 60 to facilitate the xe2x80x9coptical-leverxe2x80x9d measurement method. Further, the probes are separated from the Si substrates by etching away the wafer material beneath the probe, which is a time-consuming and costly process. Because of their relatively large size, and because much of the Si substrate is etched or otherwise destroyed during the production process, relatively few probes 50 are formed from each expensive Si wafer, thereby making the cost of each conventional probe 50 relatively high.
Another problem associated with conventional probes is that out-of-plane tips 55 must be fabricated during a separate process from that used to form holder chip 51 and cantilever section 52, and probe 50 must be mounted onto holder plate 60 at an angle relative to an underlying sample 45. Conventional methods needed to form out-of-plane tips, such as tip 55 shown in FIG. 31, add time and expense to the probe manufacturing process. Most conventional out-of-plane probe tips are either etched out of a material (e.g. Si) or they are molded (a pyramidal mold is formed by anisotropic Si etching, the mold is filled up with a material such as a metal or diamond, the mold material is removed). Further, the tip height is limited to only about 15 xcexcm, so probe 50 must be mounted onto holder plate 60 at an angle relative to an underlying sample 45 to facilitate contact between tip 55 and sample 45. To facilitate this angled probe orientation, conventional holder plate 60 is provided with an angled portion 65 to which holder chip 51 is mounted. This mounting process also takes time, and is required for each probe mounted in an SPM system.
Yet another problem associated with conventional spring probes is that, when the tip wears out, a significant amount of system downtime is required to remove and replace the worn-out probe.
What is needed is a probe structure for scanning probe systems that avoids the problems associated with conventional probes that are described above.
The present invention directed to scanning probe systems (e.g., scanning probe microscopes (SPMs)) that utilize spring probes formed from stress-engineered spring material films, and include an actuation circuit for electronically controlling the spring probe, a sensor circuit for electronically detecting the position of the spring probe, or both an actuation circuit and a sensor circuit. Each spring probe includes a fixed end (anchor portion) attached to a substrate, and a cantilever (central) section bending away from the substrate. Curvature of the cantilever section is selectively controlled during fabrication to form a long free end terminating in a tip that is located away from the substrate in an un-actuated (i.e., unbiased) state. The probe assembly, which includes the substrate, the spring probe, and optional actuation/position sensing circuits, is then mounted in a scanning probe system such that the probe tip is positioned over the surface of a sample. When the position sensing circuit is not used, a conventional measurement device (e.g., a laser beam and photosensor array) is utilized to detect tip movement while scanning.
According to a first aspect of the present invention, the actuating circuit is utilized to control the bent position of the spring probe relative to the substrate. In one series of embodiments, this actuation circuit involves electrostatic actuation utilizing an actuation electrode that is capacitively coupled to an associated spring probe. The spring probe is subsequently moved relative to the substrate by applying a differential actuation voltage to the spring probe and the actuation electrode. In one embodiment, tapered offset actuation electrodes are utilized to produce constant force, constant height, and tapping mode operations over large topographies (10s of microns), which takes advantages of the tall tip structures that can be formed by the spring probes. In other embodiments, actuation of the spring probe is performed using magnetic, acoustic and piezoelectric arrangements.
According to another aspect of the present invention, the position sensing circuit is utilized to determine the deflected position of a spring probe relative to the substrate. In one series of embodiments, this actuation circuit involves forming a resistive electrode under the spring probe, and determining the spring probe positioned by measuring the amount of current passed through the resistive element. Alternative methods, such as utilizing a piezoresistive element mounted on the spring probe, are also disclosed.
According to yet another aspect of the present invention, the various spring probe assemblies described herein are used to form inexpensive probe arrays that can significantly reduce the operating expense and down time associated with conventional scanning probe systems. As discussed above, probe tips periodically wear out, requiring system down time to replace the probe. Unlike conventional probes, multiple spring probes of the type described herein can be inexpensively fabricated on a single substrate to form a spring probe array. Further, the actuation arrangements described above can be utilized to deploy a selected spring probe while retracting the remaining spring probes of the spring probe array. Such spring probe arrays can be mounted in conventional scanning probe systems with minimal modification, and greatly reduce operating downtime (and associated expense) by allowing an operator to selectively switch between the various spring probes of the array. That is, as the tip of a spring probe wears out, the worn out spring probe is retracted and a fresh spring probe is deployed. When the substrate on which the array is formed is transparent, an optical-lever measurement system can be utilized by directing the laser beam through the substrate to strike the deployed spring probe. Alternatively, one or more of the position sensing arrangements described herein may be utilized to determine the position of the deployed spring probe. The spring probe arrays are also utilized to perform multi-point probing, multi-direction probing.